1. Field of the Invention
The present invention relates to mobile communications systems. More specifically, the present invention relates to systems and techniques for controlling the power radiated by cellular transmitters.
2. Description of the Related Art
In a cellular network a large number of mobile telephone system users, each having a cellular phone, communicate through repeaters or terrestrial stations organized in a number of cells which extend over a limited geographical area. The terrestrial stations are referred to as `base stations`.
Each mobile station (MS), e.g. cellular telephone, broadcasts a signal that is received by a base station. The signal is then relayed to a mobile switching center (MSC) which in turn routes the signal to the public switched telephone network (PTSN) and to telephone lines or other mobile stations. Similarly, a signal may be transmitted from the public switched telephone network to a mobile station via a base station and a mobile switching center.
In certain cellular systems such as GSM (Global Systems for Mobile Communication) there is a requirement to control, inter alia, the radio frequency (RF) power level of each cellular telephone transmitter in order to maximize the capacity of the system. The output power must be controlled over a wide range i.e., 30-40 dB.
One conventional technique for effecting control of the power level of each cellular phone is a closed loop approach wherein a signal is transmitted from a base station which sets the required output power level. The conventional, closed loop approach involves a feedback of the transmitter output to the input thereof via a peak detector, comparator, and variable gain amplifier.
However, this technique has been somewhat problematic inasmuch as the peak detector is generally implemented with a diode and a low pass filter. The response curve of the diode, with regard to its output voltage as a function of input power, is nonlinear over a wide range of input power levels. The nonlinearity is most extreme at low power levels. As a result, the feedback loop is nonlinear and extremely sensitive at low power levels. Inasmuch as the control voltage is typically applied using a digital-to-analog converter or `DAC`, a high resolution DAC is required to provide adequate resolution in the control of the applied voltage at the low end where the detector is most sensitive. Moreover, to ensure loop accuracy is maintained, steps must be taken to ensure the closed-loop remains stable. Methods such as changing the loop gain of the transmitter have been employed with limited success.
Hence a need exists in the art for an inexpensive system or technique for controlling the output power of a cellular telephone transmitter.